Chloritization Of Biotite Research Articles

Provenance Studies of Natural Stones Used in Historical Buildings of the Peninsula de Barbanza, Galicia, Spain (North-Western Iberia)

Traditionally, the building stones used in the architectural heritage of Galicia (Spain) during the past were mainly extracted from quarries located in the surrounding areas of the historical buildings. Thus, a great variety of monuments were built with the same type of granite but with different degrees of weathering depending on local conditions, geological context (facies), and period of construction. The main purpose of this work is to evaluate the probable origin of the construction materials of six historical buildings on the Barbanza Peninsula, Galicia (Spain), based on the degree of weathering and petrographic-mineralogical characteristics. The evaluation was performed on six different samples of granite according to the geological context in outcrops of places where there are old quarries. We used X-ray fluorescence spectrometry (XRF), X-ray diffraction (XRD), and petrographic microscopy (PM) to attempt to address whether the origin is really local to the Barbanza Peninsula or whether the stone was brought from more distant places, based on our knowledge of the geological context of the study area. Based on the chemical, textural, and mineralogical analyses obtained, the material used for construction has a local origin and comes from small quarries spread over a wide area within the Peninsula itself. Barbanza-type granite is the most abundant within the geological context of the area and also the most used as a construction material. Other types of granites identified are the Confurco Granite and other granitoids of the Noya complex. The Chemical Alteration Index (CIA) shows low to moderate weathering in the granites, fitting petrographic observations. In monuments, samples show higher concentrations of Na and K due to salt crystallization. They show microcracks due to extraction, cutting, and finishing processes and common alteration processes of the main minerals, such as sericitization of plagioclase and chloritization of biotite. This work consists of a multidisciplinary study focused on the geological perspective for the identification and preservation of historic quarries. Knowledge of the original material also presents a unique opportunity for the restoration and/or reconstruction of monuments, which allows for the maintenance of their conceptual and constructive homogeneity.

Petrography and Geochemistry of Hydrothermal Alteration in the Low-Sulfidation Epithermal Kestanelik Au Deposit, Biga Peninsula, NW Turkey

In this study, the facies and degrees of hydrothermal alteration related to the low-sulfidation epithermal Kestanelik Au deposit in the Biga Peninsula metallogenic province are identified through petrographic studies and analysis of geochemical characteristics, such as mass changes, molar element ratios, and alteration indices. The gold mineralization is located in silicified zones containing veins and stockwork veinlets of silica. In the Kestanelik Au deposit, common hydrothermal alteration is mainly found in the Permian-Upper Cretaceous Çamlıca basement metamorphics and the Eocene granodiorite, and less often in the Eocene Şahinli volcanic rocks of the Karabiga Massif on the Peninsula. Based on mineralogical and geochemical studies conducted on altered samples, four different alteration facies are defined as silicic, sericitic, argillic, and propylitic, which show remarkable differences in the behavior of REEs, Si, K, Al, Na, and Ca elements. The hydrothermal fluids that caused alteration in the Kestanelik Au mineralization and host rocks had low REE contents because of REE mobilization. In addition, the kaolinization of feldspars and micas, and the chloritization of biotite and feldspars, may have caused negative Eu anomalies. The characterization of rocks subjected to hydrothermal alteration that are most influenced by diverse K-metasomatism with the largest K gains and losses in Na–Ca is illustrated by molar element ratio plots. Depending on the intensity of K-metasomatism, gold mineralization increases with increasing K trends toward gold ore veins. In the Kestanelik Au field, the argillic, sericitic, and propylitic alteration types from the zones enclosing the Au ore veins are revealed using the Ishikawa alteration index and chlorite–carbonate–pyrite index. Mass changes in the altered rocks indicate that there are gains in Si, K, and Al, and losses in Na and Ca with the increasing intensity of alteration toward the ore veins. The results confirm the presence of silicic and K–metasomatic (sericite and argillic) and propylitic (Fe-rich chloride) alteration zoning extending from the inner regions to the outer regions, which characterize the epithermal ore systems.

Alteration Mechanism of Variscan Granite in a Project Area at the Northern Foot of the Tianshan Mountains, China

Long-term erosion of granite—a type of hard rock—by hydrothermal fluids and tectonic movement can lead to a fragmentation of the internal structure of the original rock, transfer/replacement of mineral elements, and alteration of the rock’s basic properties. Such changes can be problematic for the construction of water conservancy, hydropower, and road projects. This study adopted the altered Variscan granite in a water diversion project area at the northern foot of the Tianshan Mountains (China) as the research object, and explored the alteration mechanism using thin section identification, X-ray diffraction, major element analysis, and electron probe and oxygen isotope tests. Results showed that the lithology of the granite in the study area is mainly biotite granodiorite and biotite monzonitic granite. Reductions in both the K+ content in plagioclase and the K+ and Ti4+ contents in chlorite indicate that the alteration types within the study area are mainly clayization of feldspar minerals and chloritization of biotite. Biotite granodiorite and biotite monzonitic granite both have low δ18O values. The δ18O value of biotite granodiorite decreases with increase in the Loss on Ignition. The low-δ18O-value granodiorite due to an alteration by hydrothermal fluids transformed from glacier meltwater, groundwater, atmospheric precipitation, and magmatic water; whereas the monzonitic granite might be formed by the reinvasion of low-δ18O-value granodiorite formed in the early stage, which is remelted, assimilated, and rebalanced. The research findings provide a reference for similar research on altered granite at the northern foot of the Tianshan Mountains, and also lay a foundation for subsequent research on its physical/mechanical properties and engineering characteristics.

Retrograde Assemblages of Gneisses at Arigidi and Erusu Areas of Southwestern Nigeria

The study is aimed to unravel the presence of low-grade minerals in the migmatite terrain of Southwestern Nigeria. Mineral and textural relationship of mineral assemblages were determined using a petrographic microscope, while chemical composition was analyzed using XRF and LA-ICP-MS. Petrographic studies show the mineral assemblage, biotite + plagioclase + chlorite + epidote + quartz + muscovite + ilmenite ± calcite. There is intense alteration of both biotite and plagioclase. Textural evidence shows chloritization of biotite and epidotization of both biotite and plagioclase. Epidotization of biotite is basically at the edges of biotite while that of plagioclase in more intense at the core of grains. Primary foliation is defined by mineralogical banding of quartzo-feldspathic and mafic minerals, while secondary foliation is defined by both biotite and chlorite. SiO2 contents are high in both gneisses (> 60 wt %), while there is a high depletion of REE. The results from field observations, mineral assemblages and textural relationship suggests that deformation due to shearing could have enhanced the retrograde chloritization of gneisses. Shearing could have created a path for the influx of fluid (H2O & CO2) thus triggering alteration of both biotite and plagioclase. Hydrothermal fluids containing K+ can be said to have initiated the retrograde process which gave rise to reactions that produces secondary low grade minerals; chlorite, epidote and muscovite.

Mass transfer and element redistribution during chloritization of metamorphic biotite in a metapelite: insights from compositional mapping

Abstract Fluid-driven reactions are common in the upper crust, leading to mineral replacement and element redistribution on a kilometer to a micron scale. One common fluid-induced reaction is the chloritization of biotite. Chloritization can lead to the production of multiple phases, such as K-feldspar, titanite, and rutile, depending on local chemistry and fluid composition. Here, we compare different approaches to mass balance calculations using an altered garnet-bearing metapelite collected from Ny Friesland, Svalbard Archipelago, Norway. In the outcrop, amphibolite facies schists are cut by joints that acted as fluid pathways that led to the development of a narrow (~5 cm) alteration zone. Alteration involved the replacement of biotite by chlorite + K-feldspar ± rutile ± titanite. Mass balance calculations based on whole-rock composition and assuming immobile Zr indicate the addition of H2O, Si, Ti, Fe, or Mg and the removal of K or Ca. In contrast, mass balance calculated for the pseudomorphic replacement of Bt by Chl + Kfs ± Rt ± Ttn using X-Ray maps for three focused areas indicates the removal of Si or Ti rather than addition. Moles of product phase and elements gained or lost during this reaction varies between individual areas. This variation suggests that fluid heterogeneity and local chemistry governed the progression of the replacement reaction. All the product phases involved in the biotite breakdown are geochronometers and/or geothermometers, providing new opportunities for the petrochronology of fluid-induced reactions.

Zircon geochronology of high-grade metamorphic rocks from outcrops along the Prince Olav Coast, East Antarctica: Implications for multi-thermal events and regional correlations

This paper reports laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) zircon U–Pb ages of a suite of high-grade metamorphic rocks collected from Sinnan Rocks, Akebono Rock, Niban Rock, Gobanme Rock, Tenmondai Rock, Akarui Point, Cape Omega, and Oku-iwa Rock along the Prince Olav Coast, in the Lützow-Holm Complex (LHC), East Antarctica. The dating results indicate that a newly detected ~ 990 Ma metamorphism of garnet–sillimanite–biotite gneiss from Niban Rock. A thermal event at 931.7 ± 9.8 Ma is recorded in zircon from staurolite-bearing garnet–gedrite–biotite–chlorite gneiss in Akebono Rock. The metamorphic zircon grains in other analyzed samples provide Ediacaran to Cambrian ages. Their multi-growth textures and age populations are possibly interpreted to exhibit three metamorphic stages during >600–580 Ma, 580–550 Ma, and 550–500 Ma. Combined with previous reports, the metamorphic rocks in Cape Hinode, Niban-nishi Rock of Niban Rock, and Akebono Rock might have experienced earlier high-temperature metamorphism at ~ 990–930 Ma without younger overprinting. Extensive high-grade metamorphism during ~ 650–500 Ma is recorded from not only the granulite-facies zone in the west of the LHC but also the amphibolite-facies zone in its east. The main metamorphic episode in the LHC is likely to be subdivided into a preceding thermal event (either independent single metamorphic event or prograde stage) at pre-580 Ma, near-peak condition stage during 580–560 Ma, and subsequent retrograde stage after 550 Ma. In regional context this indicates that the assembly at the central Gondwana started with the collision of early and late Neoproterozoic terranes prior to 580 Ma, as a part of the East Africa-Antarctic Orogeny. Subsequent collisions took place among late Neoproterozoic igneous terrane, above terranes collided at pre-580 Ma, and Neoarchean terrane, which were probably driven by the Kuunga Orogeny.

Petrochemical evaluation of gahnite from volcanogenic massive sulfide deposits in Betul belt, Central India: Insight from petrography and in‐situ trace element geochemistry

Gahnite occurs in diverse rock types and metamorphosed mineral deposits. The gahnite geochemistry is being used as an exploration tool for zinc sulfide deposits. In the present study, the geochemistry of gahnite and associated minerals viz. sphalerite, garnet, sillimanite, staurolite, anthophyllite, biotite, ilmenite, and chlorite, and their petrography have been used in studying the genesis, metamorphic evolution, deposit characterization, and exploration significance of seven Palaeoproterozoic volcanogenic massive sulfides (VMS) deposits in the Betul belt in the Central Indian Tectonic Zone, India. Different textural types of gahnites are recognized in the Betul belt. Field investigations of textural and geochemical characteristics suggest that the gahnites were formed by multiple geological processes: desulfidation of sphalerite, breakdown of almandine garnet, and zincian staurolite, and chloritization of zincian biotite corresponding with regional metamorphism. The temperature conditions of gahnite formation are estimated to be ~600–650°C using empirical Ti in biotite thermometry. The elemental concentrations in gahnites vary from Zn/Fe ratio 2–4; Co 1–30 ppm; Ga 14–573 ppm and Mn 126–3422 ppm, indicating that bimodal volcanics and volcano‐sedimentary rocks as protolith. High Mn content in gahnite suggests an influx of magmatic‐hydrothermal fluid in the protolith. Variations of trace elements (Mg vs. Al/V, Ti vs. Co/Mn, Co vs. V/Ga, Mn vs. Zn/Fe) and high‐temperature of gahnite formation attest to sensu stricto VMS type mineralization in Betul belt, which is entirely different from SEDEX, Broken Hill‐type, and non‐sulfide deposits. Principal component analysis of trace elements shows the geochemical variations at a few deposits and reveals that gahnites were formed as a result of the competing effect of fS2‐fO2 and governed by bulk rock composition.

The Early Cretaceous Zaduo Granite, Eastern Qiangtang Terrane (China)—An Attempt to Constrain its Paleolatitude and Tectonic Implications

The Eastern Qiangtang Terrane is an orogenic-like belt around the Eastern Himalayan syntaxis (EHS). The deformation history of this terrane must be known to understand how the EHS region responded to the Lhasa-Qiangtang collision and the closure of the Bangong-Nujiang Ocean (BNO). Here, we present a new paleomagnetic investigation on an Early Cretaceous granite (∼126 Ma) in the Zaduo area, Eastern Qiangtang Terrane. Petrographic observations reflect crystallization from primary melts with only limited subsequent alteration (chloritization of biotite). Magnetite appears to be the dominant carrier of the characteristic remanent magnetization (ChRM) based on stepwise demagnetization of the natural remanent magnetization, supplemented by detailed rock magnetic measurements, including magnetization versus temperature, and acquisition curves of the isothermal and anhysteretic remanent magnetization. End-member modeling of those acquisition curves helped to constrain the paleomagnetic analysis. The inconsistent demagnetization behavior between alternating field (AF) demagnetization at high levels and thermal demagnetization was attributed to the development of gyroremanent magnetization in the AF demagnetization generated by fine-grained single domain magnetite. The ChRM directions from 92 granite samples in geographic coordinates yield an average of declination (Dg) of 2.6° and inclination (Ig) of 38.6° (precision parameter k = 51.4, and 95% confidence cone α95 = 2.1°). The amount of tilting of the granite is poorly constrained which makes proper correction rather tedious. We compared the expected bedding attitudes (Strikeexp = 43.1°, Dipexp = 46.1°) derived from published data (Huang et al., 1992;Tong et al., 2015) with the average observed bedding attitudes (Strikeobs = 54°, Dipobs = 32°) of the Middle-Upper Jurassic sandstones of the Yanshiping Group that was intruded by the Early Cretaceous granite. The discrepancy between the expected and measured bedding attitudes implies that the strata of the Yanshiping Group in the Zaduo area were already tilted prior to the intrusion of the ∼126 Ma Zaduo granite, which was attributed to the Lhasa-Qiangtang collision and the closure of the BNO. The collision led to a series of geological events, such as the tilting of the strata, the ophiolite emplacement, the development of a peripheral foreland basin, and the magmatic activity gap. The tilting/folding of the strata was generally delayed by the layer parallel shortening processes during the early stages of the deformation, thus suggesting an older Lhasa-Qiangtang collision (i.e., >126 Ma).

Nanoscale constraints on a fluid-induced transformation of monazite during postmagmatic alteration – A case of the Jawornik granitoid (NE Orlica-Śnieżnik Dome, Sudetes, SW Poland)

Two monazite grains altered during postmagmatic, hydrothermal processes in the Variscan Jawornik granitoid (NE Orlica-Śnieżnik Dome, Sudetes, SW Poland) were investigated. Different assemblages of secondary phases replacing and forming close to altered monazite indicate two different courses of the mineral-fluid reactions. The first monazite is surrounded by two zones comprising rhabdophane group minerals (rhabdophane henceforth) and hematite (inner zone) and allanite (outer zone) that form a concentric corona texture. The growth of secondary phases is interpreted as being related to decreasing temperatures below ca. 500 °C. The inner zone formed due to alteration of the monazite that was driven by fluid-induced coupled dissolution-reprecipitation reactions. This alteration resulted in the progression of partial replacement by submicron layers of rhabdophane along the monazite surface and pseudomorphic replacement of a substantial part of the monazite by rhabdophane and hematite. The presence of the hematite suggests oxygen fugacity above the hematite-magnetite buffer. Alteration of the second investigated monazite is related to dissolution induced by a K-bearing alkali fluid, partial replacement by thorianite (ThO2), and overgrowth by K-feldspar with minor titanite formed during the chloritization of biotite and the alteration of feldspars. These results demonstrate that the local character of mineral-fluid reactions can take an entirely different course at the microscale, within a distance of several hundreds of microns. The assemblages of secondary phases replacing two investigated monazite grains indicate that the alteration processes are related to moderate temperature (<500 °C) hydrothermal processes assigned to upper greenschist facies conditions. The absence of secondary apatite (close to the first monazite) and secondary REE- and P-bearing phases (close to the second monazite) indicates the high mobility of the elements removed by the fluid, in contrast to the low mobility of the Th taken up by secondary rhabdophane or thorianite crystallizing at the monazite surface. The presence of rhabdophane group minerals and thorianite in hydrothermal secondary phases assemblages replacing monazite can be used as indicators of the local character of the environment at the microscale.

Experimental alteration of granitic rocks: Implications for the evolution of geothermal brines in the Upper Rhine Graben, Germany

Geothermal energy exploitation in the Upper Rhine Graben (URG) chiefly has targeted faults and fractures within or connected with the crystalline basement, where hot fluids of c. 200 °C circulate at depths of c. 5 km. Formation fluids of the crystalline basement are highly saline, NaCl-dominated brines, whereas shallow crystalline basement water (< 500 m) is only weakly mineralized. In order to contribute to a better understanding of redistribution of elements in fluids and alteration phases during fluid-rock interaction processes we conducted leaching and alteration experiments with granites and monzonites from the Schwarzwald at 70 °C (rock powder, pure water) and 200 °C (texturally intact rock cylinders, 2m-NaCl), respectively, inspired by the P-T-x conditions reported for the URG.The trace element concentrations of the leachates are hereby related to the composition and stability of minerals in the rocks and can be directly linked to the proposed and observed dissolution processes at the different temperatures. In experiments with pure water at 70 °C, representing the recharge or infiltration conditions, water-rock interaction mobilized K, Cl, Fe, Mn, Mg, Cu, Zn, Cr, Sc, V, and Li (dissolution of biotite/chlorite) and Si, Al, K, Ca, Na, Ba, Sc, Pb, Sr, Rb, and Eu (dissolution of plagioclase, K-feldspar, and quartz) into the fluid. According to calculated activity diagrams, alteration at these temperatures may result in the formation of illite, kaolinite, Ca-beidellite, and stilbite. In experiments at higher temperatures of 200 °C, according to reservoir conditions, the net output of most elements was increased, but especially of those contained in feldspar, i.e. K, Ca, Al, Si, Ba, Rb, Cs, and Pb, suggesting that fluid-rock interaction is dominated by feldspar dissolution. This alteration is followed by albitization of feldspar surfaces, chloritization of biotite, and precipitation of illite and hematite. In Si-poor samples, analcime occurs in and around porous reaction zones, in agreement with calculated activity diagrams.The amount of dissolution mainly depends on the temperature and the mineralogy of the rock types, in particular on the anorthite-component of plagioclase. Certain elements (e.g., Na, Fe, Mg, Al) were subsequently incorporated into the solids precipitated during the experiments and thus cannot be considered as conservative.

Elemental behavior during chlorite alteration: New insights from a combined EMPA and LA-ICPMS study in porphyry Cu systems

Chlorite is a common alteration product of fluid-rock interaction in hydrothermal deposits, and it's compositional variations provides useful information about the physicochemical conditions of formation, but element behavior during the chloritization process is still not well understood. Chlorite from two large porphyry Cu deposits, Atlas (Philippines) and Xiaokelehe (NE China) have been chosen to evaluate the geochemical variations with a focus on mass transfer during alteration of precursor minerals.Conversion of two biotite to one chlorite is the main mechanism for the chloritization of biotite. Iron, Mg, Al, and Ni are mostly retained in chlorite, whereas Co, Ga, Mn, and Zn are commonly transferred to chlorite from the hydrothermal fluid, and Sc, Sr, Si, V, Li, K, Nb, Ba, Rb, Ti, Cl, Na, Sn, and Cu from the biotite mostly do not enter chlorite during chloritization. Chlorite alteration of hornblende formed probably through the processes of dissolution-precipitation. Iron are mostly retained in the chlorite, whereas Li, Cu, Ni, Zn, Co, Al, and Ga in the chlorite are at least in part derived from hydrothermal fluid, and Mg, Mn, V, Si, Ca, Zr, Nb, Sn, Cl, REE, Y, Na, Ti, Sr, K, Sc and Ba originally in the hornblende mostly enter hydrothermal fluid, titanite or epidote, rather than chlorite during chloritization. The MgO, FeOT, MnO, Zn, Li, Sc, V, Co and Ni concentrations, and FeOT/MgO ratio of chlorite are strongly influenced by those of the precursor minerals. Moreover, although formed in the same porphyry system, the MnO, Zn, Li and Cu concentrations and FeOT/MgO ratios of chlorite formed from biotite and hornblende are markedly different.Given that Fe/Mg ratios of chlorite are controlled by the precursor minerals in wall rock, the Fe/(Fe + Mg) correction on the well-established chlorite thermometers during hydrothermal alteration may need to be reconsidered. Aluminum, Ga, and Ti in chlorite are not controlled by precursor minerals, but by formation temperature of chlorite, so we can use these elements of chlorite to map out the thermal structure of porphyry system which is related to individual mineralized porphyry intrusion, which can help to identify the centers of hydrothermal systems.

Chlorite alteration in porphyry Cu systems: New insights from mineralogy and mineral chemistry

Chlorite, an important mica-like clay mineral formed in a variety of geological environments, is sensitive to bulk rock composition and physicochemical conditions during formation, and thus an ideal mineral to investigate the nature of fluid-rock reactions and mass transfer during hydrothermal fluid interaction. Samples of chlorite from alteration systems around three large porphyry Cu deposits, Tuwu (NW China), Atlas (Philippines) and Xiaokelehe (NE China), were used to investigate element transfer from pyroxene, amphibole, and biotite to chlorite.By calculating the chlorite formation temperatures using the chlorite empirical thermometer, the temperature ranges for biotite, hornblende, and pyroxene chloritization in this study were broadly the same (around 270 °C). Contacts are gradational between biotite and chlorite grains along biotite cleavage but abrupt between hornblende/pyroxene and chlorite. Concentrations of K2O, Na2O, TiO2, and SiO2 decreased gradually from biotite to chlorite, whereas MgO, FeOT, Al2O3, and MnO concentrations increase. Different forms of hydrothermal titanite grains occurred commonly in the biotite, hornblende, and pyroxene chloritization processes in the porphyry systems studied, and titanite formed during chloritization of biotite had higher Al2O3 and F contents compared to titanite that formed during chloritization of hornblende and pyroxene. Given the high Al2O3 and F contents in biotite, this suggested the concentrations of these components in titanite may be controlled by the composition of the precursor minerals. The FeOT and MgO contents of chlorite correlated with the composition of the precursor mineral, whereas their TiO2 and Al2O3 contents did not, with all chlorite having similar contents. This suggested that FeOT and MgO contents of chlorite were more likely controlled by the precursor minerals, whereas TiO2 and Al2O3 were probably controlled by the similar formation temperature of chlorite without obvious influence of precursor minerals. Thus, TiO2 and Al2O3 contents of chlorite could be more suitable for use as a mineral geochemical vector for hydrothermal center in porphyry deposits.

Mg isotope fractionation during partial melting of garnet-bearing sources: An adakite perspective

Garnet that has a distinctly light Mg isotopic composition compared to other mafic minerals is commonly present in deep crustal and mantle sources. Whether partial melting of these garnet-bearing sources can lead to significant Mg isotope fractionation or not remains poorly understood. In order to investigate Mg isotope fractionation during partial melting of garnet-bearing sources, we report Mg isotopic data of 17 well-characterized low-Mg adakitic rocks (LMA) from the Dabie orogen, central China. These LMA are known for derivation from mafic sources leaving eclogitic residua with variable abundance of garnet. Our data together with previously published results show that fresh Dabie LMA have homogeneous Mg isotopic compositions, with an average δ26Mg of −0.22 ± 0.08‰ (2SD, N = 18), except that two samples display higher δ26Mg (−0.05‰ to +0.01‰) due to alteration during sericitization and chloritization of biotite. The lack of correlation between δ26Mg and (Dy/Yb)N of the Dabie LMA indicates an unresolvable residual garnet effect. Given that Mg isotopic compositions of the Dabie LMA are identical with local mafic basement within the current precision, Δ26Mgmelt-cpx is estimated to be around −0.02‰. Limited isotope fractionation of the Dabie LMA could be attributed to the dilution effect of residual clinopyroxene co-existing with garnet. The estimated Δ26Mgmelt-cpx is then applied to predict the possible isotope fractionation that may occur during hydrous and dehydration melting of subducted oceanic crust. With a lower melting temperature and more abundant garnet in the residua, slab-derived melts may have δ26Mg higher than their sources by up to +0.36‰, which, however, can be easily erased by later melt-mantle interaction. In this regard, Mg isotopes can serve as a powerful tracer of recycled supracrustal materials.

Pre‐orogenic upper crustal softening by lower greenschist facies metamorphic reactions in granites of the central Pyrenees

AbstractPre‐kinematic greenschist facies metamorphism is often observed in granites and basement units of mountain belts, but rarely dated and accounted for in orogenic cycle reconstructions. Studying pre‐kinematic alteration is challenging because of its usual obliteration by subsequent syn‐kinematic metamorphism often occurring at conditions typical of the brittle–ductile transition. It is, however, to be expected that pre‐kinematic alteration has major implications for the rheology of the upper crust. In the 305 Ma‐old Variscan basement of the Bielsa massif (located in the Axial Zone of the Pyrenees), successive fluid–rock interaction events are recorded in granites below 350°C. Combined microstructural and petrographic analysis, low‐T thermobarometry and in situ U–Th/Pb dating of anatase, titanite and monazite show extensive pre‐orogenic (pre‐Alpine) and pre‐kinematic alteration related to feldspar sericitization and chloritization of biotite and amphibole at temperatures of 270–350°C at 230–300 Ma. This event is followed by a second fluid–rock interaction stage marked by new crystallization of phyllosilicates at 200–280°C and is associated with the formation of mylonitic shear zones and fractures parallel to the shear planes. U–Pb anatase and monazite ages as well as the microtextural relationships of accessory minerals suggest an age for this event at 40–70 Ma, consistent with independent regional geology constraints. The Variscan basement was therefore softened at late to post‐Variscan time, at least 150–200 Ma before the main Alpine shortening while Alpine‐age compression (c. 35–50 Ma) leads to the formation of a dense net of mylonites. The associated deformation, both distributed at the scale of the Bielsa massif and localized at decametric scale in mylonitic corridors, precedes the strain localization along the major thrusts of the Axial Zone. The Bielsa massif is a good example where inherited, pre‐orogenic upper crustal softening controls the deformation patterns in granitic basement units through low‐grade metamorphic reactions.

Finite pattern of Barrovian metamorphic zones: interplay between thermal reequilibration and post-peak deformation during continental collision—insights from the Svratka dome (Bohemian Massif)

The Barrovian inverted metamorphism of the Svratka dome developed within two nappes derived from the Brunia continent that was thrust beneath the Moldanubian orogenic root. The metamorphism increases from biotite–chlorite zone in the basement to very closely spaced staurolite, kyanite and sillimanite zones at the top of the nappe pile. The sequence of mineral growth, chemical zoning of garnet, and pseudosection modelling indicate prograde paths from 4.5 kbar/510 °C to 5.5 kbar/540 °C in the garnet zone, from 6 kbar/530 °C to 7 kbar/600 °C in the staurolite zone, and from 3.5 kbar/510 °C to 8.5 kbar/650 °C in the kyanite zone. The age of monazite inclusions in garnet and staurolite is interpreted to reflect prograde metamorphism at 338 ± 7 Ma and 336 ± 7 Ma, respectively. An older matrix monazite crystal is interpreted as dating prograde crystallization at 345 ± 7 Ma, whereas a younger monazite group records recrystallization at/or down to 334 ± 7 Ma. While these petrological and geochronological data are consistent with data from an inverted metamorphic sequence of the southern Thaya dome, the spacing and distribution of metamorphic zones, nappe thicknesses, and late structures are different in the two domes. An antiformal stack of imbricated basement sheets and the extreme attenuation of metamorphic isograds at the top of the nappe pile in the Svratka dome are explained by a relatively cold overthrusting Moldanubian domain, formed mainly of middle orogenic crust. The homogeneous thickening of the hinterland-dipping basement duplexes and the regular spacing of metamorphic isograds in the Thaya dome are explained by a hot overriding Moldanubian domain, which in this region has a high proportion of exhumed lower orogenic crust and formed a hot mid-crustal channel.

Exotic garnet–clinopyroxene–K-feldspar granulites from the Chepelare shear zone, Central Rhodope massif, Bulgaria: implications for high-pressure granulite facies metamorphism

Garnet–clinopyroxene–K-feldspar granulite occurs as a thick layer or boudin within the variegated rocks of the Chepelare shear zone in the Central Rhodope massif, Bulgaria. It consists of several domains: mesocratic homogeneous matrix (clinopyroxene–plagioclase–K-feldspar–quartz ± amphibole), porphyroblastic garnet, K-feldspar and clinopyroxene, and strongly foliated fine-grain bands (chloritized biotite–chlorite–prehnite–albite ± epidote). The origin and nature of the matrix mineral association is still unclear. The peak porphyroblast association forms at the expense of plagioclase from the matrix at higher pressure. The fine-grain deformation zones channel the lattermost fluid infiltration. The clinopyroxene-garnet and Zr-in-titanite thermometry give temperatures higher than 790–860 ºC at 2 GPa and, with thermodynamic modeling, suggests crystallization at ~1.8–2.1 GPa and temperature of ~850 ºC in HP granulite field for the porphyroblast granulite association.

Permian–Triassic red-stained albitized profiles in the granitic basement of NE Spain: evidence for deep alteration related to the Triassic palaeosurface

Extensive areas of the Variscan granitic basement in NE Spain display profiles of red-stained albitized facies characterized by albitization of Ca-plagioclase, chloritization of biotite and microclinization of orthoclase, along with the alteration of igneous quartz to secondary CL-dark quartz. These profiles have a geopetal structure beneath the Triassic unconformity, with a very intense and pervasive alteration in the upper part that progressively decreases with depth to 150–200 m where the alteration is restricted to the walls of fractures. The red albitized facies contains secondary maghemite and hematite that indicate oxidizing conditions. Dating of microclinized orthoclase and secondary monazite that have formed in the red-stained albitized facies yielded K–Ar and U–Th–Pbtotal ages of 240 and 250 Ma, respectively, suggesting that the alteration developed during the Permian–Triassic period. The geopetal disposition of the red albitized profile with respect to the Triassic unconformity, its large regional extent, and the fracture-controlled alteration in the lower part of the profile indicate groundwater interaction. The δ18O values of albitized plagioclase (+ 11‰), microclinized orthoclase (+ 13‰), and secondary CL-dark quartz (+ 12‰) suggest that the alteration temperature was about 55 °C. This “low” temperature suggests that the alteration occurred during interaction of the granitic rocks with Na-rich fluids below a surficial weathering mantle on the Permian–Triassic palaeosurface. The latter is possibly related to Triassic evaporitic environments in long-lasting, stable landscapes in which Na-rich solutions infiltrated deep regional groundwaters.

Petrology and U–Pb zircon age of the Variscan porphyroclastic Rand Granite at the southeastern margin of the Central Schwarzwald Gneiss Complex (Germany)

The Variscan Rand Granite as defined in this paper is a deformed I-type biotite granite that intruded along the southern-to-southeastern margin of the Central Schwarzwald Gneiss Complex. Former K-feldspar megacrysts (now porphyroclasts) of this K–Mg-rich alkali-calcic granite frequently show zonal crystallographic arrangement of mineral inclusions and are enclosed in a matrix of plagioclase, K-feldspar, quartz, biotite, apatite, zircon, and magnetite. Minor sphene and allanite are mostly altered. K-feldspar is orthoclase with perthitic exsolutions. Myrmekite is common and typically replaces marginal K-feldspar. Both feldspars show cataclastic and incipient ductile deformation that took place within the stability field of biotite (≥ 400 °C) as proven by grey varieties of Rand Granite with stable biotite. At most places, however, the Rand Granite shows a reddish colour, caused by late-stage chloritization of biotite and formation of hematite within K-feldspar. Furthermore, plagioclase became partially altered to sericite. This hydrothermal alteration took place at temperatures below the stability of biotite (< 400 °C). In situ ion probe U–Pb dating on zircon gave a concordant age of 330.9 ± 4.8 Ma (2σ), interpreted as the intrusion age of the Rand Granite. A large number of younger concordant to slightly discordant zircon ages between 309 and 90 Ma are interpreted to be due to episodic Pb loss during hydrothermal alteration. The Rand Granite apparently does not contain zircon domains older than the intrusion age and, furthermore, shows relatively high Zr contents (247–358 µg/g). These characteristics suggest high magma temperatures of at least 850–900 °C. The granitic magma most probably resulted from remelting of K-rich mafic to intermediate rocks in the middle crust at H2O-undersaturated conditions. Low Sr/Y ratios suggest a garnet-free residuum, which is only possible at pressures below ~ 0.9 GPa.

U-Pb (LA-ICP-MS) age of detrital zircons and the sources of terrigenous sediments of the Ipsit suite, Karagass series (Sayan segment of the Sayan-Baikal-Patom belt)

The petrographic, lithogeochemical and U-Pb (LA-ICP-MS) geochronological studies were carried out to investigate the terrigenous rocks sampled from the lower part of the Ipsit suite of the Karagass series (Sayan segment of the Sayan-Baikal-Patom belt). These rocks include sandstones, aleurite sandstones and aleurolites, and their mineral compositions are close to that of arkose. Most of the studied rock samples show petrographic features typical of the epigenetic changes at the stage of catagenesis: regeneration of quartz clastic grains, pelitization of potassium-feldspar clastic grains, occurrence of clay-hydromica aggregate, sericitization of plagioclase, chloritization of biotite, and silicification of dolomite pieces, and occurrence of authigenous tourmaline. The above was confirmed by the analysis of the concentrations of petrogenic elements in the studied rocks from the lower part of the Ipsit suite. The analysis results show that the concentrations of K2O are elevated, while the concentrations of Na2O are relatively very low, which may be due to the redistribution of these elements during epigenetic transformations. According to the classification by genetic types on the basis of the system of petrochemical modules, the rocks of the lower part of the Ipsa suite are of the petrogenic nature. The acidic igneous rocks are dominant in the source area, as evidenced by the presence of granitoid and quartzite fragments in the clastogenic component, as well as the set of accessory minerals typical of the igneous rocks of the acidic composition, and the distribution pattern of rare and trace elements. According to the U-Pb (LA-ICP-MS) dating of detrital zircons from the aleurite sandstone sampled from the lower part of the Ipsit suite, the zircons are exclusively of the Archean-Early Proterozoic ages. Such ages correlate with the age of the granitoids of the Sayan complex and the felsic volcanites from the Maltsev layer of the Elash series (Biryusa block). Furthermore, the detrital-zircon age spectra of the aleurite sandstone of the lower part of the Ipsit suite are identical to the detrital-zircon age spectra of the terrigenous rocks from the underlying strata of the Shangulezh and Tagul suites of the Karagass series. This study suggests that sedimentation of the Ipsit suite of the Karagass series took place due to the influx of detrital material from the southern part of the Siberian craton into the sedimentation basin, and the acidic igneous rocks of the Biryusa block were one of the main sources of detrital material.

Mineralogy and titanite geochronology of the Caojiaba W deposit, Xiangzhong metallogenic province, southern China: implications for a distal reduced skarn W formation

The Caojiaba tungsten deposit (19.03 Mt@ 0.37 wt% WO3) is hosted by skarn along the contact between clastic and carbonate rocks in the Xiangzhong Metallogenic Province of southern China. The deposit is characterized by an early prograde skarn containing low andraditic garnet (Ad0.7–21.9) and hedenbergitic pyroxene (Hd52.9–77.3) overprinted by a retrograde biotite–chlorite assemblage and then by quartz–scheelite veins, similar to well-studied reduced tungsten skarns worldwide. Scheelite has low MoO3 (0.01–0.16 wt%), and ore commonly contains up to 1.5 ppm Au and up to 0.33 wt% Sb. Sensitive high-resolution ion microprobe (SHRIMP) U–Pb analyses of hydrothermal titanite coexisting with scheelite in three skarn ore samples provide ages between 206 ± 5 Ma and 196 ± 3 Ma (2σ). Our new ages demonstrate that the tungsten mineralization took place at Caojiaba between 206 and 196 Ma, overlapping the 228–201 Ma emplacement age of granitic rocks in the Xiangzhong Metallogenic Province. Mineralogical and geochronological evidence collectively indicates that Caojiaba is a distal reduced W skarn deposit. The 226–196 Ma granite-related W mineralization recognized throughout the province has a possible link with the widespread Sb–Au mineralization in the region.